Thermal spraying processes are employed in various industries to apply protective coatings to surfaces of components that will be subjected to harsh operating conditions, such as high temperatures and impaction by abrasive materials. In these thermal spraying processes, the coating materials are melted or vaporized, and sprayed onto the substrate surface that is to be coated. Typically, a “feedstock” material in wire or powdered form is heated by an electrical means, such as plasma or arc, or a chemical means, such as a combustion flame, to a molten or semi-molten state, and accelerated toward the substrate surface by a plasma gas jet and/or another high pressure air jet. Thermal spraying processes are superior to other coating processes such as electroplating because the substrates may be covered with a relatively thick coating over a large surface area in a shorter processing time.
Plasma transferred wire arc (PTWA) thermal spraying is one type of thermal spraying process for depositing a coating of the feedstock material to both external and internal substrate surfaces. PTWA thermal spraying is particularly applicable for use in coating the concave inner surfaces of the cylinder bores of internal combustion engines. Heat and abrasion resistant coatings on the cylinder bores enable the use of aluminum engine blocks without the need for heavy cast iron sleeves as are otherwise required to withstand the heat generated during the combustion cycle. In many PTWA bore spray torches, a single conductive wire is used as the feedstock for the system. A supersonic plasma jet formed by a transferred arc between a non-consumable cathode and the feedstock wire melts and atomizes the wire, and an additional forced gas jet transports the stream of molten droplets onto the surface the cylinder bore as the torch rotates within the cylinder bore to apply an even coating to the entire surface.
PTWA bore spray torches in various forms are known in the art. For example, U.S. Pat. No. 5,296,667, issued to Marantz et al. on Mar. 22, 1994, and entitled “High Velocity Electric-Arc Spray Apparatus and Method of Forming Materials,” teaches a torch assembly employed in a manner suitable for depositing a uniform coating on the surface of a concave surface such as a bore. The torch assembly is mounted on a rotating member to allow rotation concentrically with respect to the bore by means of a motor drive. At the opposite end, the rotating member is mounted on a stationary end plate and formed with an insulating wire feed conduit extending through its rotation axis. The torch assembly is mounted on the radius of the rotating member so that a plasma jet extends toward the insulating wire feed conduit. A wire is fed on the central axis of the bore through the wire feed conduit, and gas and electrical connections to the torch assembly are brought through the stationary end plate to and through the rotating member to the torch assembly. A transferred-arc plasma melts off the tip of the wire as it is continuously fed into the plasma jet, and the molten droplets are atomized and propelled by the plasma stream towards the inner wall of the bore. As the rotating member and the transferred-arc plasma torch assembly are rotated, a coating is deposited uniformly on the wall of the bore. While the deposit is being formed by the rotational movement, the assembly is reciprocated axially within the bore to cause the deposit to form all along the circumference of the bore as well as the length of the bore.
In another example, U.S. Pat. No. 5,468,295, issued to Marantz et al. on Nov. 21, 1995 and entitled, “Apparatus and Method for Thermal Spray Coating Interior Surfaces,” discloses a method of thermal spray coating a surface with a metal coating material including the provision of a nozzle about a thermal spray coating apparatus, such as a two wire electric-arc apparatus. The nozzle includes a plurality of ports facing generally radially inwardly towards a coating material particle stream, such as an atomized molten metal stream of a two-wire arc thermal spray apparatus. The ports sequentially receive a deflecting gas flow, such that the direction moves circumferentially about the axis of the particle stream. The deflecting gas entrains the coating material and carries it radially to the surface of the part to be coated or the nozzle assembly. The plurality of valve ports allows for coating of a cylindrical surface without rotating the end of the assembly. A method and apparatus for simultaneously coating a plurality of bores of a main body, such as an engine block, includes a plurality of deflecting nozzles which are simultaneously inserted in each of the bores.
To ensure even and complete coating of the interior surfaces of the cylindrical bores, the torch assembly and the nozzles in the exemplary patents, as well as other similar arrangements where rotation of a sprayer or a spray about a rotational axis occurs, coincidental alignment of the rotational axis with a longitudinal axis of the bore allows the sprayer to deposit an even and complete coating over the interior surface. Once the relative positions and alignments of the sprayer and the cylindrical bores are established, the sprayer may be repetitively inserted into the bores as the engine blocks travel past the spraying station on the production line or are otherwise cycled through the spraying station. From time to time, however, it is necessary to reestablish the spatial relationship between the sprayer and the cylindrical bores. For example, the alignment may be compromised when the sprayer hits an engine block or other obstruction, or when the sprayer must be removed from the assembly for maintenance or replacement. At present, no robust mechanism or method exists for aligning these types of sprayers to the part that is being sprayed. Without proper alignment, the surface may not be properly coated, and excessive defects can occur such as hot spots at the ends of the bores with insufficient coatings to withstand the heat generated and the abrasions occurring during the combustion cycle. In view of this, a need exists for an improved apparatus and method for aligning a sprayer with a bore or other concave surface to be coated as the sprayer rotates about its rotational axis.